1. Field of the Invention
This invention relates generally to the field of laser diodes and, more particularly, to circuitry for delivering a light beam of controllable intensity from a laser diode.
2. Discussion of the Prior Art
A laser diode is a device which emits coherent light whose intensity is approximately proportional to the current flowing through the diode over a 20:1 dynamic range. A laser diode's light intensity may be effectively modulated by time-varying the current through the diode. Such modulation is useful in various applications including, for example, imaging systems where a laser diode is used as a light source for the pixel-by-pixel recording of continuous tone images.
A laser diode's operating characteristics are temperature-dependent and may vary widely in response to changes in the ambient temperature or self-heating. Such temperature changes may cause significant changes in the intensity of emitted light even though the current through the diode is constant.
A servo loop circuit may be used to control and stabilize the light emitted by a laser diode. A portion of the light emitted by the diode is used as a feedback signal and applied to a photodetector. The magnitude of a signal produced by the photodetector is related (i.e., proportional) to the intensity of the light emitted by the laser diode. By comparing the signal produced by the photodetector with an input signal (reference), a difference or error signal is obtained which may be used to adjust the current flowing through the laser diode and, in turn, the intensity of light emitted.
One problem which arises with such feedback circuits is that the loop gain decreases when the current flowing through the laser diode is relatively small (i.e., low light intensity), because the laser diode is operating in a non-linear region. In this region, relatively large changes in the magnitude of the current through the laser diode produce only small changes in the intensity of light emitted by the laser diode (and thus small changes in the feedback signal). Consequently, it is desirable to integrate the error signal to force the error to zero, which has the effect of trading off loop gain for bandwidth.
However, conventional integrating feedback circuits may not provide sufficient bandwidth for applications in which the input signal varies rapidly over a wide dynamic range. One such application, referenced above, is continuous tone imaging, which typically requires extremely rapid modulation of the light emitted by the laser diode over a large dynamic range.
An additional problem arises when a photodiode (typically a PIN diode is chosen) is used as the photodetector in the feedback loop. Because of the diode junction capacitance the voltage drop across the photodiode is a function of frequency, significant error may be introduced into the loop where that voltage drop is used as a feedback signal and there is significant variation in frequency.
In one prior arrangement, the current from the diode passes through a resistor and the resulting voltage across the resistor is applied to a unity gain amplifier. The output of this amplifier is the feedback signal for the servo loop.
The output of the unity gain amplifier is also fed back to the photodiode, ideally to maintain a constant zero (AC) voltage across the photodiode (in order to cancel the effects of the photodiode's junction capacitance). However, the amplifier has a finite gain bandwidth product, which means that at higher frequencies a non-zero voltage will appear across the photodiode and error will be introduced into the system. In addition, at high frequencies the unity gain amplifier requires a high input current and is generally noisy.